Top emitting, electroluminescent component with frequency conversion centres

ABSTRACT

In order, in the case of an OLED, to improve the coupling-out efficiency, a top emitting electroluminescent component ( 100 ) is proposed, comprising a substrate ( 110 ), a first electrode ( 120 ) closest to the substrate, a transparent electrode ( 140 ) located at a distance from the substrate, and at least one light-emitting organic layer ( 130 ) arranged between the two electrodes. The component according to the invention is distinguished by the fact that a coupling-out layer ( 150 ) is arranged on that side of the second electrode which is remote from the at least one organic layer, said coupling-out layer comprising conversion centres ( 151 ) which partly absorb the light emitted by the at least one organic layer and emit it again with an altered frequency. In this way, directional light penetrating into the coupling-out layer is emitted again in non-directional fashion, so that a part of the light which would be captured by means of total reflection without the influence of the frequency conversion centres within the coupling-out layer can indeed leave the coupling-out layer.

The invention relates to a top emitting, electroluminescent componentand to a method for producing it.

BACKGROUND OF THE INVENTION

In recent years a high demand has developed for ever smaller,space-saving, light and cost-effective display modules and displays forfast and adequate visualization of data. In the field of flat screensfor notebooks, mobile telephones and digital cameras, LCDs (liquidcrystal displays) are still predominant at the present time. However,they have some disadvantages, such as the great angular dependence ofcontrast and colours, slow response times in the event of picture andcontrast change, and a low efficiency caused by a multiplicity offilters and polarizers, so that comparatively high energies have to beused for obtaining the required luminous intensity. In this respect,there is a high demand for small high-resolution and power-savingscreens with improved presentation properties. Displays based on organiclight-emitting diodes (OLEDs) represent an alternative to LCDs sincethey themselves comprise light-emitting pixels and, consequently, do nothave any backlighting. They can be produced in the form of a film, forexample, flexibly and thin with low production costs and can be operatedwith a comparatively low expenditure of energy. With their low operatingvoltage, the high energy efficiency and also the possibility ofproducing planar emitting components for the emission of any desiredcolours, OLEDs are also suitable for application in illuminationelements.

OLEDs are based on the principle of electroluminescence, in whichelectron-hole pairs, so-called excitons, recombine with emission oflight. For this purpose, the OLED is constructed in the form of asandwich structure in which at least one organic film is arranged asactive material between two electrodes, positive and negative chargecarriers being injected into the organic material, a charge transport ofholes or electrons to a recombination zone in the organic layer takingplace, where a recombination of the charge carriers to form singletand/or triplet excitons occurs. The subsequent radiative decompositionof the excitons causes the emission of visible useful light that isemitted by the light-emitting diode. In order that this light can leavethe component, at least one of the electrodes must be transparent. Thistransparent electrode is generally composed of conductive oxides,referred to as TCOs (transparent conductive oxides). The starting pointin the production of an OLED is a substrate, to which the individuallayers of the OLED are applied. If the electrode nearest to thesubstrate is transparent, the component is referred to as a “bottomemission OLED”; if the other electrode is embodied in transparentfashion, the component is referred to as a “top emission OLED”. The sameapplies to the case of fully transparent OLEDs, in which both theelectrode between the substrate and the at least one organic layer andthe electrode located at a distance from the substrate are embodied intransparent fashion.

As explained, light is generated in the active zone or emission zone ofthe component through radiative recombination of electrons and defectelectrons (holes) by means of excitonic states. The different layers ofthe OLEDs, for example the transparent electrodes and the at least oneorganic layer, generally have a different refractive index, which isnaturally greater than 1. In this respect, not all of the photonsgenerated can leave the component and be perceived as light since totalreflections can occur at the various interfaces within the component orbetween the component and air. Moreover, a part of the light generatedis also absorbed again within the component. Depending on the design ofthe OLEDs, besides the propagation of external modes, on account of thetotal reflection described above, optical substrate and/or organic modesform (that is to say propagation of light in the substrate, thetransparent electrode and/or the at least one organic layer). If theelectrode nearest to the substrate is not transparent (top emissionOLED), besides external modes it is merely possible for modes topropagate in the at least one organic layer and/or the electrode locatedat a distance from the substrate, which are referred to jointly asorganic modes. Only the external optical modes can be perceived as lightby the observer, that proportion of the total luminescence generatedwithin the component which is made up by said modes being approximately20%, depending on the design of the OLED. In this respect, there is aneed to couple these internal optical modes, that is to say organic and,if appropriate, substrate modes, out of the component to a greaterextent in order to achieve a highest possible efficiency of the organiclight-emitting component.

In order to improve the coupling-out efficiency, a multiplicity ofmethods and configurations, particularly for bottom emitting OLEDs, areknown which are concerned with the coupling-out of the optical substratemodes. For this purpose, the article “30% external quantum efficiencyfrom surface textured, thin-film light-emitting diodes” by I. Schnitzer,Appl. Phys. Lett., Volume 63, page 2174 (1993), proposes roughening thesurface of the substrate, whereby the occurrence of total reflection atthe interface between substrate and air is avoided to a considerableextent. This roughening may be achieved for example by etching orsandblasting the substrate area remote from the organic portion. Thepaper “Improvement of output coupling efficiency of organiclight-emitting diodes by backside substrate modification”, by C. F.Madigan, Appl. Phys. Lett., Volume 76, page 1650 (2000), describesapplying a spherical pattern to the rear side of the substrate surface.Said pattern may comprise an array of lenses, for example, which isapplied to the substrate by adhesive bonding or lamination. The article“Organic light emitting device with an ordered monolayer of silicamicrospheres as a scattering medium” by T. Yamasaki et al., Appl. Phys.Lett., Volume 76, page 1243 (2000), proposes applying microspheres madeof quartz glass to the surface of the substrate in order to improve thecoupling-out of light in the case of an OLED. Said microspheres may alsobe arranged beside the OLED. Furthermore, it is also known to produceperiodic strictures having a period length in the region of thewavelength of the light emitted by the OLED between substrate and firstelectrode, said periodic structure continuing into the optically activelayer of the light-emitting diode. The geometry specified ultimatelyresults in Bragg scattering that increases the efficiency of thecomponent, see J. M. Lupton et al., Appl. Phys. Lett., Vol. 77, page3340 (2000). The published German patent application DE 101 64 016 A1furthermore relates to an organic light-emitting diode in which the atleast one organic layer has different partial regions having differentrefractive indices. On account of the deflection at the phase boundarieswithin the organic portion, fewer photons remain captured as a result ofwave-guiding losses in the layer than in the case of homogeneous layers.

In addition to this utilization of intrinsic inhomogeneities in theactive organic layer, it is furthermore known to introduce impuritiessuch as nanoparticles to the organic electroluminescent material, thusmaking it possible to avoid waveguide effects within the organicportion, see for example “Enhanced luminance in polymer composite lightemitting devices”, by S. A. Carter et al., Appl. Phys. Lett., Vol. 71(1997). Said nanoparticles may be composed for example of TiO₂, SiO₂ orAl₂O₃ and be embedded in a polymeric emitter material, such as MEH-PPV.

In addition to bottom emitting OLEDs, top emitting OLEDs areincreasingly gaining in relevance since they have advantages over theformer for specific applications. If both the two electrodes and thesubstrate are transparent, it is possible to provide anelectroluminescent component which emits in its entirety, that is to saytowards the top and bottom. If the substrate does not have to betransparent as in a top emitting OLED, besides glass it is also possibleto use many other substrates which make it possible, for example, forthe component to be flexible, that is to say pliable. Furthermore, metalfoils, silicon wafers or other substrates with silicon-based electroniccomponents and also printed circuit boards may also serve as substratesin a top emitting electroluminescent component of this type.

SUMMARY OF THE INVENTION

The invention is based on the object of improving the coupling-outefficiency for light in a top emitting, electroluminescent component ofthe generic type. This object is achieved in a surprisingly simplemanner in terms of a device just by means of a component having thefeatures of claim 1. In this case, the top emitting electroluminescentcomponent according to the invention, which may be formed in particularas an organic light-emitting diode device, comprises a substrate, afirst electrode nearest to the substrate, a second, transparentelectrode located at a distance from the substrate, and at least oneorganic layer arranged between the two electrodes, at least one of theorganic layers being a light-emitting layer. The component isdistinguished according to the invention by virtue of the fact that acoupling-out layer is arranged on that side of the second electrodewhich is remote from the at least one organic layer, said coupling-outlayer comprising frequency conversion centres which partly absorb thelight emitted in the at least one organic layer and emit it again withan altered frequency.

The invention's configuration of the top emitting, electroluminescentcomponent enables the coupling-out efficiency thereof to be increased bymore than 100%, depending on the specific embodiment, in comparison withthe conventional configuration of such a component without acoupling-out layer. Furthermore, the coupling-out layer can also performeven further functions besides the function specified.

The invention is based on the idea, with the provision of a coupling-outlayer on that side of the second electrode which is remote from theorganic layer or layers, of influencing the propagation of the opticalmodes within the organic layers and the transparent electrode such thatthe coupling-out efficiency is increased. For this purpose, conversioncentres are provided in the coupling-out layer, which conversion centrespartly absorb the light emitted by the at least one organic layer andemit it again with an altered frequency. In this way, directional lightpenetrating into the coupling-out layer is emitted again innon-directional fashion, so that a part of the light which would becaptured by means of total reflection without the influence of thefrequency conversion centres within the coupling-out layer can indeedleave the coupling-out layer.

According to the invention, the coupling-out layer comprising frequencyconversion centres is thus designed for reducing total reflection withinthe component. Although this is accompanied by a change in the frequencyof the light, this can be tolerated in many applications for topemitting, electroluminescent components. This applies both to monochromeor multi-chrome displays configured according to the invention and toillumination components designed according to the invention.

Advantageous embodiments are specified in the subclaims.

Depending on the embodiment, the coupling-out layer may bear directly onthe second electrode or be connected to the latter, but may also bespaced apart from the latter at least in sections. If the coupling-outlayer and the transparent electrode form a common interface, or areconnected to one another, this results in a particularly effectivecoupling of the organic modes into the coupling-out layer. In specificapplications, however, it may also be expedient if the coupling-outlayer is spaced apart from the second electrode at least in sections,but the distance should be less than approximately the wavelength of thelight emitted by the at least one organic layer.

It may be expedient if the coupling-out layer comprises a matrix, inparticular for the purpose of applying the coupling-out layer a matrixthat can be brought into solution, in which the frequency conversioncentres are introduced. The coupling-out layer may be formed exclusivelyby the matrix, and the frequency conversion centres introduced therein.Said matrix may comprise in particular a photoresist in which thefrequency conversion centres are embedded. Ultimately, the matrix is notrestricted to organic materials but rather may also comprise or beformed from inorganic substances.

In principle, all substances which, through absorption and reemission oflight, are able to transform directional light into non-directionallight can be used as frequency conversion centres in the coupling-outlayer. As the person skilled in the art is aware, the frequency changein this case is a side effect that has to be accepted since the physicalprinciple of the conservation of energy applies to the operationdescribed. In this respect, the light can be emitted by the frequencyconversion centres only with a lower frequency. With regard to a leastpossible loss of energy, it is particularly advantageous in this case ifthe energy levels of the frequency conversion centres are configured insuch a way that the frequency difference between the absorbed light andthe emitted light is the smallest possible, for example less than 100nm. It is particularly expedient if said difference is less than 30 nm.By specifically setting the emission spectrum of the frequencyconversion centres, it is possible to bring about a specific colourimpression in the case of the component according to the invention.

It should be pointed out that, depending on the embodiment, thefrequency conversion centres may have a discrete absorption level and adiscrete emission level; on the other hand, it may also be the case thatthe centres absorb or emit over a predetermined frequency range, forexample a few to several tens of nanometers.

It may be expedient if the frequency conversion centres are provided byat least one dye or a dye mixture. This may have both an organic and aninorganic construction. Typical dimensions of dye molecules lie between1 Å and 2 nm.

By way of the setting of the dye concentration, it is possible to setthe quantity of the frequency conversion and thus the extent of thedirectional conversion of the light in the coupling-out layer. Theinventors have found a dye concentration of less than 1% by volume to beparticularly expedient. This makes it possible to achieve a situation inwhich, although a certain part of the otherwise totally reflected lightis directionally converted within the coupling-out layer, a predominantpart of the light emitted by the at least one active organic layer isnot absorbed by the frequency conversion centres. This last would havethe consequence of further reducing the degree of coupling-out of theentire component through the coupling-out layer, which is undesirable.In this respect, according to the invention, the essential parameters ofthe coupling-out layer such as, in particular, dye substance,concentration, light absorption and coupling-out layer thickness are tobe coordinated with one another in such a way as to produce an increaseddegree of coupling-out.

Expediently, the thickness of the second electrode may be less than 200nm, in particular less than 80 nm, thus resulting in a particularlyeffective coupling of the light from the organic portion into thecoupling-out layer since the evanescent field is not greatly attenuatedby the second electrode. In this case, the refractive index of thecoupling-out layer may advantageously be established to be greater thanthe refractive index of the nearest organic layer arranged between theelectrodes. It is particularly advantageous if the thickness of thesecond electrode is even smaller, in particular approximately 40 nm.

In order to avoid a total reflection of the light at the interfacebetween the second electrode and the coupling-out layer if the lightenters the coupling-out layer from the electrode, it may be providedthat the refractive index of the coupling-out layer is greater than therefractive index of the second electrode. In this case, it may beexpedient if the refractive index of the coupling-out layer lies between1.3 and 2.3, in particular between 1.6 and 2.0. As a result, the organicmodes are completely or substantially coupled into the coupling-outlayer.

In principle, it may be stated that top emitting, electroluminescentcomponents may also be equipped with the coupling-out layer according tothe invention if they have a plurality of organic layers. As specifiedin particular in the published German patent application DE 102 15 210A1, it may be advantageous if, besides the light-emitting organic layer,other organic layers are also arranged between the two electrodes. Inthe case of a non-inverted construction, such a general structure of acomponent has the following layers:

-   1. substrate,-   2. first electrode, hole-injecting anode,-   3. p-doped, hole-injecting and transporting layer,-   4. thin hole-side intermediate layer made of a material whose energy    level of the HOMO (highest occupied molecule orbital) matches the    energy levels of the HOMOs of the layers surrounding it;-   5. light-emitting layer,-   6. thin electron-side intermediate layer made of a material whose    energy level of the LUMO (lowest unoccupied molecule orbital)    matches the energy levels of the LUMOs of the layers surrounding it,-   7. n-doped electron-injecting and transporting layer,-   8. second electrode, electron-injecting cathode.

In the case of an inverting construction of the component, the followinglayers result:

-   1. substrate,-   2.a) first electrode, electron-injecting cathode,-   3.a) n-doped, electron-injecting and transporting layer,-   4.a) thin electron-side intermediate layer made of a material whose    energy level of the LUMO (lowest unoccupied molecule orbital)    matches the energy levels of the LUMOs of the layers surrounding it,-   5.a) a light-emitting layer,-   6.a) thin hole-side intermediate layer made of a material whose    energy level of the HOMO (highest occupied molecule orbital) matches    the energy levels of the HOMOs of the layers surrounding it,-   7.a) p-doped hole-injecting and transporting layer,-   8.a) second electrode, hole-injecting anode.

According to the invention, then, provision is made of an additionallayer—designated as coupling-out layer—for increasing the degree ofcoupling-out.

As specified in the published patent application DE 102 15 210 A1, thehole transport layer may be p-doped with an acceptor like organicmaterial and the electron transport layer may be n-doped with a donorlike organic material.

The inventors of the present invention have additionally found that theelectron transport layer may also be n-doped with an alkali metal. Theseconfigurations result in an increased conductivity, so that thetransport layers may have higher layer thicknesses than usual incomparison with undoped layers (typically 20 to 40 nm) without theoperating voltage being drastically increased. In this respect, in thecase of a non-inverted construction of the component according to theinvention, it may be expedient if a further organic layer is arrangedbetween the coupling-out layer and the emitting organic layer, whichfurther organic layer is an electron transport layer that is n-dopedwith a donor like organic material and has a thickness of between 20 nmand 2 μm, in particular a thickness of between 30 nm and 300 nm. In thecase of an inverted construction of the component, said further organiclayer is a hole transport layer that is p-doped with an acceptor likeorganic material and has a thickness of between 20 nm and 2 μm, inparticular a thickness of between 30 nm and 300 nm. It goes withoutsaying that, in accordance with the above-specified general structure ofa component according to the invention, another electrode and also,under certain circumstances, a block layer are furthermore arrangedbetween the coupling-out layer and the emitting organic layer.

For the sake of completeness, it should be pointed out that, dependingon the embodiment of the component according to the invention, aninverting or respectively non-inverting design may not encompass all ofthe layer types specified above, furthermore provision may also be made,however, of other layers such as, for example, a thin (less than 10 nm)contact-enhancing layer between the electron transport layer and thecathode and/or between the anode and the hole transport layer. For thesubsequent process steps, in particular for the application of thecoupling-out layer in a manner adjoining the second electrode oradjacent to the latter, it may be favourable to provide a thick dopedcharge transport layer between the light-emitting organic layer and thecoupling-out layer, which represents a protection for the light-emittinglayer during the production of the coupling-out layer.

Expediently, the coupling-out layer has a thickness of between 0.05 μmand 1000 μm, in particular between 0.5 μm and 100 μm.

It may be expedient if the coupling-out layer is configured in such away that it not only increases the light coupling-out efficiency, butalso simultaneously represents a protection for the layers arrangedbetween the electrodes against mechanical loading, electromagneticradiation, particle radiation, moisture, air and/or chemical influences.In this way, the coupling-out layer additionally affords anencapsulation or protection function, which is advantageous particularlyin the case of display and illumination applications.

The inventors have found that it is expedient if the transmittance ofthe coupling-out layer at the wavelength of the light emitted by the atleast one organic layer is greater than 0.4 and the transmittance of thecoupling-out layer at the wavelength of the light emitted by thefrequency conversion centres is greater that 0.6. In this case, thetransmittance τ of the coupling-out layer is determined according to therelevant formula τ=e^(−(αd)), where α specifies the absorptioncoefficient and d specifies the thickness of the coupling-out layer.Setting the described parameters of the coupling-out layer ultimatelyhas the effect of producing an optimized additional coupling-out oflight energy.

In terms of a method, the invention achieves the above object by meansof a method for producing a top emitting, electroluminescent component,designed in particular as an organic light-emitting diode device, havingthe steps of:

-   -   providing a substrate,    -   applying a first electrode nearest to the substrate,    -   applying at least one light-emitting organic layer,    -   applying a second, transparent electrode located at a distance        from the substrate, and    -   applying a coupling-out layer comprising frequency conversion        centres on that side of the second electrode which is remote        from the at least one organic layer.

This coupling-out layer may be fashioned by means of one or more of theknown techniques for applying thin layers. In particular, thecoupling-out layer may be applied wet-chemically to the secondelectrode. In this case, the coupling-out layer may be formed from amatrix material and mixed with the frequency conversion centres, themixture being applied wet-chemically. For the purposes of processing, asolvent can be added to the matrix material. Said solvent may on the onehand serve for the purposes of wet-chemical application of thecoupling-out layer or only enable the frequency conversion centres to bemixed with the matrix material. Moreover, it is also possible to providea dispersant for mixing the frequency conversion centres with the matrixmaterial.

It is particularly advantageous to use a positive photoresist as matrixmaterial, which is dissolved in a corresponding solvent and mixed with adye, the mixture being applied to the transparent electrode by means ofa spin- or roller-coating method, by way of example.

A method for applying the coupling-out layer which is particularlyadvantageous because it is simple to carry out and is gentle on thecomponent according to the invention consists in processing a film thatis laminated or adhesively bonded onto the second electrode, the filmbeing provided with the frequency conversion centres.

Furthermore, the coupling-out layer may also be applied by means of athermal vapour deposition method. In this case, it may be advantageousif the matrix material and the substance comprising the frequencyconversion centres are vapour-deposited in a thermal co-vapourdeposition method in the gas phase.

As already explained above, a transport layer having a thickness ofbetween 30 nm and 300 nm and comprising an organic doping or an alkalimetal may advantageously be applied, so that the coupling-out layer cansubsequently be applied to the top thin contact layer (trans-parentelectrode) wet-chemically or by thermal vapour deposition withoutdamaging the light-emitting organic layer.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention is explained below by the description of a plurality ofembodiments with reference to the accompanying Fig., in which

FIG. 1 shows a conventional top emitting OLED in a schematicillustration,

FIG. 2 shows a top emitting OLED designed according to the invention inaccordance with a first embodiment in a schematic illustration.

FIG. 1 shows a schematic sketch of the construction of a conventionalelectroluminescent, top emitting component 100. In the examplespecified, the electrode 120 nearest to the substrate 110, whichelectrode is designated hereinafter as first electrode, is embodied as areflective metal layer. A plurality of organic layers, specified asorganic layer structure 130 in the Fig., are applied to the firstelectrode. Said layer structure comprises at least one organicelectroluminescent layer. The layer structure 130 is adjoined by asecond electrode 140 composed of a transparent material, for example aconductive oxide.

When a voltage is applied between the two electrodes, charge carriers,that is to say electrons from one side and holes from the other side,are injected from the contacts into the organic layers situated inbetween, whereupon electron-hole pairs form in the active zone, whichrecombine with emission of light. An exemplary emission point isdesignated by the numeral 131 in the Fig. The light propagatesproceeding from this emission location, this being indicated byindividual arrows in the Fig. As can be discerned, a reflection of thelight and/or a transmission into the next layer is effected at theinterfaces between two layers. The light which remains (beam OM1) withinthe component, here within the layer structure 130 and/or the electrode140, is referred to as the organic mode, and the light which leaves thecomponent (beams EM1, EM2) is referred to as external modes. Since eventhe organic layers have an absorption coefficient not equal to zero forthe light generated within the layers, said light is absorbed in thecourse of propagation in the longitudinal direction with respect to thelayer.

The invention then commences here with a particular configuration of thecomponent in order to increase the coupling-out efficiency in the caseof a top emitting component. For this purpose, in one embodiment, acoupling-out layer having frequency conversion centres is provideddirectly on the second electrode. Such an embodiment is illustrated in aschematic sketch in FIG. 2. Since the number of organic layers that emitlight by means of electroluminescence is of secondary importance to thepresent invention, these layers are actually only specified as layerstructure 130 in FIG. 2. An electrode 120 is applied to the substrate110, said electrode being adjoined by the organic layer structure 130,in which the light is generated. This is adjoined by the secondelectrode 140, to which an additional layer, the coupling-out layer 150,is applied according to the invention. The latter layer has frequencyconversion centres 151.

The component according to the invention as illustrated in FIG. 2 can beproduced in different ways, depending on the specific embodiment. In onekind of embodiment, the coupling-out layer is applied wet-chemically bymeans of a printing method (inkjet printing, screen printing,flexographic printing, pad printing and further relief printing,intaglio printing, planographic and screen process printing methods), orblade coating, spin-coating, dip-coating, roll-coating, spraying, etc.Prior to the application, a dye for example, in particular a laser dye,may in turn be admixed depending on the embodiment.

Depending on the embodiment, e.g. one or more of the following materialsis or are used for the coupling-out layer, said materials being presentas solution, emulsion and/or dispersion during processing, depending onthe application method utilized. After application to the component,these materials form the matrix of the coupling-out layer, e.g. by meansof the evaporation of the solvent or by optical curing:

-   -   Polymer solutions such as e.g. solutions of polyfluorenes or        polystyrenes in organic liquids, e.g. aromatic solvents, such as        xylene, toluene, anisole, trimethylbenzene and the like.    -   solutions of organic non-polymeric layer-forming materials such        as solutions of organic glasses, e.g. ortho-terphenyl or        1,3,5-tri-alpha-naphtyl-benzene in aromatic solvents, e.g.        xylene,    -   a monomer or a mixture of monomers which polymerise after        application, such as methyl methacrylates or allyl diglycol        carbonate or derivatives thereof which are polymerized        thermally, chemically or in a photo initiated manner after        application,    -   a monomer or a mixture of monomers which are linked by        polyaddition after application, e.g. polycarbonates,    -   optical adhesives,    -   photoresist,    -   transparent or semitransparent adhesives such as chemically        curing adhesives (e.g. 2-component adhesives), thermally curing        adhesives (e.g. acrylates, epoxy resins) or UV-curing adhesives        such as acrylates or epoxy resins,    -   transparent thermoplastics such as low density polyethylene,        polycarbonates and polyurethanes,    -   thermosetting plastics such as phenolic resins or melamine        resins,    -   emulsions such as aqueous or organic or fluoro-organic emulsions        comprising e.g. polyacrylate, polyvinyl alcohol or polyvinyl        acetate,    -   clear coatings such as alkyd resin coatings, nitro and nitro        combination coatings, two-component coatings such as        polyurethane coatings, water-dilutable coatings, synthetic resin        coatings and acrylate coatings,    -   collagenic proteins such as gelatine, cellophane or celluloid,    -   dispersions such as polymer dispersions (e.g. titanium dioxide        particles and polyvinyl acetate in water) and    -   solutions or dispersions comprising inorganic materials such as        salt solutions.

Depending on the matrix material used, frequency conversion centresadapted thereto are introduced into the matrix material. Such centreshave characteristic absorption bands and emission bands and may haveboth an inorganic and an organic structure. In this respect, thesuitable frequency conversion centre material may be selected in amanner tailored to the emission spectrum of the electroluminescentmaterial used and the matrix material used.

Organic or inorganic dyes or else inorganic microcrystals may be used asfrequency conversion centres,

Exemplary organic dyes are:

-   -   many known laser dyes, e.g. nile blue, cresyl violet,        sulforhodamine B, rhodamine B, fluorescein 548, coumarins,    -   DCJTB, DCM    -   many emitters known from OLEDs, e.g.        1,4-bis(9-ethyl-3-carbazovinylene)-9,9-dihexylfluorene;        4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl; Alq3

Exemplary inorganic microcrystals are:

-   -   microcrystals of suitable phosphors    -   microcrystals of fluorescent emitters such as ZnSe, ZnS

In terms of the method, in the case of the component 100 illustrated inFIG. 2, proceeding from a substrate 110 comprising a flexible,non-transparent plastics material, a first metal electrode 120 isvapour-deposited and patterned. Afterwards, an organic layer structuremade of Alq₃ as electroluminescent material is processed on in aconventional manner. A transparent electrode made of a TCO material isin turn applied thereon. Finally, the co-vaporization of silicon dioxideand rubrene is effected, rubrene being vapour-deposited in aconcentration of 1% by volume.

The light generated in the organic layer structure 130 leaves thecomponent only via the coupling-out layer 150 since the bottom electrode120 and the substrate 110 are not transparent. The refractive index ofthe coupling-out layer, with a value of 2.0, is slightly higher than theindex of the organic layer structure 130 and of the electrode 140. Inthis respect, a significant part of the light that is otherwise capturedas an organic mode in the optical layer structure and the electrode 140is coupled-out into the layer 150. A part of the electroluminescentlight in the coupling-out layer can leave the latter towards the top,see the beams EM1 and EM2 in FIG. 2. On account of the—in comparisonwith air—high refractive index of 2.0, however, total reflection occursat the air/coupling-out layer interface for a part of the light from theorganic layer structure. This part of the light is reflected back andforth within the coupling-out layer between the interfaces. In thecourse of these reflections, the light also impinges on the dye centres151 arranged in the coupling-out layer. These absorb theelectroluminescent light and emit the energy again through the emissionof light with a slightly lower frequency. What is crucial for theinvention here is that the absorbed light was directional, while the dyemolecules emit photons again in non-directional fashion. What isachieved in this way is that at least a part of the light that isotherwise captured in the coupling-out layer 150 is emitted by thecolour centres 151 in directions in which no total reflection occurs,see the beam KM1 in FIG. 2. Consequently, the component according to theinvention emits both primary light as electroluminescent light andsecondary light that has been emitted by the frequency conversioncentres. This increases the efficiency of the component according to theinvention. The ratio of secondary light to primary light may be between11:1 and 1:20, for example depending on the embodiment.

A top emitting component of this type can be used e.g. as anillumination element with an increased efficiency.

A further class of top emitting electroluminescent components accordingto the invention is produced by applying the coupling-out layer byco-vaporization, in particular thermal co-vaporization of matrixmaterial and conversion centre material. When using this method, whichis presented only by way of example) the following matrix materials, inparticular, are used:

-   -   organic layers, such as e.g. monomers, which are        vapour-deposited and subsequently polymerized, such as methyl        methacrylate (MMA), acrylic acid,    -   organic layers comprising small molecules such as aromatics,        aliphatics, heterocyclic compounds, ketones; for example        tetrakis-diphenylaminospirobifluorene (spiro-TAD),        triscarbazolyltriphenylamine (TCTA), bathophenanthroline        (Bphen).

Most of these materials presented for the coupling-out layer aredistinguished not only by their transparency in the visible spectralrange but also by a refractive index that is greater than or equal tothat of the electroluminescent layer structure. In this respect, thelight generated in the active organic layer is coupled particularlyeffectively from the organic layers into the coupling-out layer of thecomponent according to the invention and is coupled out fi-m therethrough the colour centres specified out of the structure. Since most ofthese materials of the coupling-out layer, although they are transparentin the visible spectral range, are highly absorbent in the UV range,such coupling-out layers afford the organic layers not only protectionagainst moisture and air but also against UV radiation.

LIST OF REFERENCE SYMBOLS

-   100 OLED component-   110 Substrate-   120 First electrode-   130 Organic layer/layer structure-   131 Emission point-   140 Second electrode-   150 Coupling-out layer-   151 Conversion centres-   EM1, EM2 External mode-   OM1 Organic mode-   KM1 Converted light

1. Top emitting, electroluminescent component comprising a substrate, afirst electrode closest to the substrate, a second, transparentelectrode located at a distance from the substrate, and at least oneorganic layer arranged between the two electrodes, wherein at least oneof the organic layers is a light-emitting layer, and wherein acoupling-out layer designed for improving the coupling-out of light isarranged on that side of the second electrode which is remote from theat least one organic layer, said coupling-out layer comprising frequencyconversion centers which partly absorb the light emitted in the at leastone organic layer and emit it again with an altered frequency, whereinthe coupling-out layer is an organic matrix in which the frequencyconversion centers are provided, wherein the frequency conversioncenters are provided by at least one dye, wherein the dye concentrationin the coupling-out layer is less than 1% by volume, wherein thetransmittance degree of the coupling-out layer at the wavelength of thelight emitted by the at least one organic layer is greater than 0.4 andthe transmittance of the coupling-out layer at the wavelength of thelight emitted by the frequency conversion centers is greater than 0.6.2. Component according to claim 1, wherein the coupling-out layer is onthe second electrode and is connected to the latter.
 3. Componentaccording to claim 1, wherein the coupling-out layer is at a distancefrom the second electrode, the distance being less than 500 nm. 4.Component according to claim 1, wherein the dye has dye molecules havinga dimension of between 1 Å and 2 nm.
 5. Component according to claim 1,wherein the dye is an organic dye.
 6. Component according to claim 1,wherein the light emitted by the frequency conversion centers isfrequency-shifted to a lower frequency with respect to the lightabsorbed by the centers, the energy levels of the frequency conversioncenters being configured in such a way that the frequency differencebetween the absorbed light and the emitted light is less than 30 nm. 7.Component according to claim 1, wherein the thickness of the secondelectrode is less than 80 nm, and the refractive index of thecoupling-out layer is greater than or equal to the refractive index ofthe nearest organic layer arranged between the electrodes.
 8. Componentaccording to claim 1, wherein the refractive index of the coupling-outlayer is greater than or equal to the refractive index of the secondelectrode.
 9. Component according to claim 1, wherein the refractiveindex of the coupling-out layer lies between 1.6 and 2.0.
 10. Componentaccording to claim 1, wherein the organic layer that is arranged betweenthe electrodes and is nearest to the coupling-out layer is a holetransport layer which is p-doped with an acceptor like organic materialand has a thickness of between 30 nm and 300 nm.
 11. Component accordingto claim 1, wherein the organic layer that is arranged between theelectrodes and is nearest to the coupling-out layer is an electrontransport layer which is n-doped with a donor like organic material andhas a thickness of between 30 nm and 300 nm.
 12. Component according toclaim 1, wherein the organic layer that is arranged between theelectrodes and is nearest to the coupling-out layer is an electrontransport layer which is n-doped with an alkali metal and has athickness of between 30 nm and 300 nm.
 13. Component according to claim1, wherein the coupling-out layer has a thickness of between 0.5 μm and100 μm.
 14. Component according to claim 1, wherein the coupling-outlayer is constructed in such a way that it represents a protection forthe layers arranged between the electrodes and the electrodes themselvesagainst mechanical loads, electromagnetic radiations, particle radiationsuch as α/β radiation, moisture, air and/or chemical influences.